Mixing labels or mixing tags are used in Ultra High Frequency (“UHF”) electronic article surveillance (“EAS”) interrogation systems and are based on a frequency mixing principle. Typically, mixing tags include a diode attached to a dipole antenna. The tag is tuned at a specific microwave frequency (fuhf), such as for example 915 MHz. The frequency range can be chosen from hundreds of Megahertz up to several Gigahertz by adjusting the antenna's dipole length and the diode's junction capacitance. The lower the operating microwave frequency, the longer the dipole length is required, and the larger the capacitance.
However, there are inherent limitations with deactivation devices when having to deactivate tags having diodes. For example, U.S. Pat. Nos. 4,318,090 and 4,574,274 provide UHF mixing tags that use diode non-linear elements and means for direct contact or non-direct contact but with limited range. The breakdown characteristics of the diode requires that a substantial current be driven through diode in order to achieve deactivation, thus resulting in the direct contact with the tag in order to supply sufficient electrical energy to the diode to cause it to be destroyed, thus deactivating the tag. This results in an impractical deactivation system since it is not always possible or economically feasible to be limited to this type of “contact” deactivation. Thus, tag designs of this type are ineffective in situations where deactivation of the tag takes place from a distance, i.e., where the deactivation device is not in contact with the tag. Other prior art deactivation systems (such as the system disclosed in U.S. Pat. No. 5,608,379) have attempted to avoid this problem by adding switches and other hardware devices to the deactivation system. This proves to be costly and cumbersome and results in relatively low deactivation distances for a considerably large magnetic field source.
Neither of the aforementioned attempted solutions solves the problem of how to effectively deactivate EAS tags at a substantial distance without the need for the deactivation device to be in direct contact with the EAS tag and without the need to provide additional deactivation elements to the EAS tag. The inherent characteristics of diodes with their predictable non-linear behavior render EAS deactivation systems that utilize these types of EAS tags ineffective when it comes to deactivating tags from a distance.
Therefore, what is needed is a new EAS tag using a non-linear element that exhibits very low level breakdown characteristics so that reliable deactivation at a considerable distance is achieved.